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Supercharger | Turbo

Discussion in 'Technical Q&A' started by traimpz348, Dec 13, 2005.

  1. traimpz348

    traimpz348 Formula 3

    Apr 13, 2004
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    What's the difference between a Supercharger and a Turbocharger?
     
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  3. mk e

    mk e F1 World Champ

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    They both increase hp the same way. They both use a compressor to increase the pressure in the intake manifold, which increases the density of the charge and makes a bigger fire when it burns in the cylinder. The difference is how the compressor is driven. With a supercharger the compressor is mechanically driven by the engines crankshaft, usually by a belt, but it could be gears or direct-drive as well. A turbocharger uses a turbine to drive the compressor and the engines exhaust gas powers the turbine.

    They both have advantages and disadvantages. A supercharge can provide instantaneous response because it is direct driven and produces boost over the engines entire operating range, but makes the engine less efficient. It’s very simple and makes a nice driving car usually at the cost of reduced mileage over just increasing the size of the engine.

    A turbo can actually make the engine more efficient, but because they are driven off the exhaust flow, they suffer from lag (when you push the throttle, you what more power but the turbo can’t react until there is more exhaust and the turbine has time to accelerate). Also it is not possible for 1 size turbo to work across the engines entire operating range. Normally they size it to work well at full power, and settle for no boost at low power, more complicated systems use 2 or more different size turbos to try to even out the boost.
     
  4. traimpz348

    traimpz348 Formula 3

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  5. Kram

    Kram Formula Junior

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    Here’s something that will interest you about turbocharging:
    During W.W.II the germans built some weird motors. Part of their problem was that they didn’t use octane boosters in their gas, with the result that for a given weight of motor they ran into a horsepower ceiling before the Allies did. They also derived all their avgas from coal, which gave them limited supplies of the stuff. As a consequence they built some diesel aircraft engines.

    Now diesel engines are heavy, needing a stronger block than a gasoline counterpart (because they rely on compression for ignition, not on an electric spark). Aircraft must be light in order to fly, and lighter still in order to fly well, but in times of war, particularly one you’re losing, you end up using everything you’ve got.

    One prototype aircraft engine consisted of a turbocharger mounted on top of a diesel engine- fine so far - only the motor had no conrods or crankshaft. There were heads at both ends of the cylinders, leaving the pistons to float, or rather shuttle back and forth as the fuel detonated, first on one end, then on the other. The air and exhaust ports were slits in the side of the cylinders, just like a two-stroke lawnmower. So, without a crank, how did the engine produce power? Answer: Off the back of the turbocharger.

    Basically the diesel engine had been reduced to becoming a pump that fed a non-combusting jet engine with hot gasses. The engine’s design owed it’s existence to the engineer who, seeing the need for a jet, was instructed to make it run on diesel rather than kerosine and to build the whole thing without super high temperature components. He did, but the power to weight ratio was too poor for the engine to make it into production.

    As a footnote the lack of BHP pushed the Germans into making proper jets (they had a bomber flying in 1941, but the engines were junk after 25 hours) whereas the Allies, with additives that made their gas harder to burn, could increase the compression ratio and control the detonation so that more force was focused on the piston head, resulting, ultimately, in aircraft that were breaking the sound barrier with their propellor tips.
     
  6. John Harry

    John Harry Formula Junior

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    Kram: What a fascinating piece of info! The human mind is endlessly ingenious (although not always in the service of humanity).
     
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  8. bill365

    bill365 F1 Rookie

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    Hi Mark,
    I had always thought that the turbos were less efficient. Both have efficiency losses in regards to the method of driving the pumps. Add, that the pre-heating of the gasses running through the HOT turbo then required the addition of intercoolers to gain back some of that loss. Both incur the same losses of the heating associated with the compression of a gas (if boost is similar), but that remains the same for both. You could regain some of that loss with an intercooler, but that could be employed in both systems.

    Bill
     
  9. mk e

    mk e F1 World Champ

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    No, not really. What you’re saying is sort of true, some of the heat in the intake charge does come from conduction though the housing form the exhaust, but not much and insulation can make it nearly zero. Most of the heat come from the inefficiency of the compressor. A good centrifugal compressor (like found on a turbocharger) will be 65-70% efficient. That means that 30-35% of the work that is put into it goes to making heat. The rest of the heat comes just compressing the gas, following T2= T1(P2/P1)^((k-1)/k).

    With superchargers, it’s the same thing. The standard roots type supercharger is about 40% efficient, a screw type is 60%-65%, a centrifugal is 65%-70 just like a turbo. There are reasons why eack are the best choice, depending on the application.

    A roots type is inexpensive, durable, does no internal compression, and is a positive displacement pump. What that means is that if the application is for low boost, say 5 psi of less and not required during normal operation, the low efficiency isn’t a problem with fuel consumption, the temperature rise won’t be a problem and adding a simple by-pass valve will let the compressor spin using almost no power when boost isn’t required. Also, because it positive displacement, just like a piston engine, the flow characteristics match the engine well giving pretty uniform boost over almost the full rpm range of the engine. For all these reasons many OEM applications go with a roots compressor.

    The screw type compressors are basically a higher performance version of the roots type. They cost more to build, but have higher efficiency, so they can deliver higher boost without the intake charge being too hot to deal with. They are also positive displacement and produce uniform boost, but tend to have there highest efficiency near redline vs the engine that hits peak efficiency (torque peak) some 2000 rpm below redline. That causes the boost to rise a bit up near redline, which tends to help flatten the torque curve and help the engine pull better to redline. The down side is that you have to plan for peak boost which is up at redline, so the boost is slightly less everywhere else even though the engine is built to take more. Also they have internal compression, so the compressor is doing some work any time it’s spinning. That means that a by-pass valve will only mostly unload the compressor, a magnetic clutch or similar device is required to completely unload it to get mileage up which requires sophisticated controls to maintain drivability. The higher output OEM systems use this type of compressor.

    A centrifugal supercharger gives the worst of both worlds between supercharging and turbocharging. The reason is that a centrifugal compressor is not positive displacement, meaning it pumps a fixed amount with each revolution (like and engine), the flow is exponential with RPM. That causes the supercharger to actually be a flow restriction at low rpm (say below ½ redline) and then has rapidly increasing boost up to redline. A turbocharger uses a wastgate to flatten the boost curve by diverting exhaust and thereby altering the rpm of the compressor. In supercharger form, that is not possible since it is driven directly off the crank. There have been schemes to use torque converters or other variable ratio drives to solve the problem, but no one has ever really made them work well….even though they are common in the aftermarket because they are cheaper than a screw type and make good peak hp for advertisement.

    Ok, now engine efficiency. An engines efficiency is basically controlled by compression ratio, or more accurately expansion ration. In a naturally aspirated engine the compression and expansion ratios are the same, but that is not true in a boosted engine. In a supercharged engine the supercharger is pre-compressing the intake charge increasing the compression ratio but does nothing to increase the expansion raito. What that means is that on an engine with 15psi boost and a 10:compression ration, the true compression is 2x10=20:1, but the expansion is still 10:1. So the exhaust exists the engine before it is fully expanded, meaning you put work into compression that you never get back, it’s wasted by dumping the exhaust at elevated pressure.

    In a turbocharged engine the compressor increases the compression ratio just like in a supercharged engine, the turbine increases the expansion ratio too. So the extra work that is put into compression is recovered. The net effect is an engine operation at a higher compression ratio and therefore higher efficiency than even a naturally aspirated engine.

    Adding an intercooler to either a supercharged or turbocharged engine reduces the power required to compress the charge, increasing efficiency….but it also allows more boost, so while trucks and trains have them to increase mileage, car normally get then to increase power.

    It has been common on trains to have both superchargers and turbochargers on the same engine. They use the supercharger in the yard to get power. The supercharger is on a clutch and when they get out of the yard and the rpm up, they disengage the supercharger and run on a turbo to increase mileage. VW is now offering a euro car that does the same thing, it’s a 1.4 liter with a supercharger for low rpm power and a turbo for mileage and high rpm power.
     
  10. ROGUE GTS

    ROGUE GTS Formula Junior

    May 24, 2004
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    One thing that hasn't been addressed is pumping efficiencies between SC and Turbos. It takes A LOT of power to spin a SC. To the tune of 5-11% depending on the unit, boost psi, #'s of air, drive config etc. It's not uncommon for a SC to eat 50hp just to spin it's self. It's hard to imagine till you see the hoops SC guys go through just to prevent belt slippage. When you have a 12 rib belt pulled super tight and it's still slipping, there is some serious force there. Hence why they go to cog or gear drives.

    Turbos on the other hand have very little pumping loss, and will always make more HP if properly setup. But in the end everything is a compromise. The guys who complain about turbo lag have obviously built a setup to run hard at higher rpm ranges. Build a SC around that and it won't be making a ton of boost off idle either. At the same time a Roots or TS blower will make massive power right off idle, but a comparably sized turbo will absolutely decimate it when the power comes on.

    Turbo lag, while not a "myth" isn't always a huge issue. It is very dependant on engine size, rpm range, and vehicle application. I would venture to say turbos are the prefered method of FI for 90% of the racers out there in just about any given application.

    About the only time I can see them having a distinct disadvantage is in bracket racing. Only because races are won and lost in +/- .001 a slight deviation in spool charastericts could cause a problem.
     
  11. bill365

    bill365 F1 Rookie

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    The compression of the gasses, if being taken to the same level of boost, will create the same amount of heat. Intercoolers are used for the same reasons on both, to allow the higher compression of the gasses.

    The HP required to spin the SC is taken direcly off of the belt, yes. What about the losses created by having to spin the turbo being a restriction in the exhaust flow? The inefficiency of not being a optimized exhaust flow + the additional heat being transferred through the turbo to the gasses you are trying to compress, do detract from the turbos effectiveness.

    Please explain how a turbo provides an expansion ratio, that would not be present in the use of an SC.

    The head of engineering at Mercedes Benz was once asked if they were planning on adding turbos on any gas powered models in the future. He laughed at the interviewer and replied, "No, turbos are for diesels." To this date, I don't remember MB ever offering a Turbo on a gas powered vehicle.

    Bill
     
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  13. ROGUE GTS

    ROGUE GTS Formula Junior

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    LMFAO, Uhhhh, have you missed the current 600 and AMG 65 series cars? Might want to check that out... yip they are all twin turbo V12's.

    There is a small bit of backpressure in the turbo but when properly sized but not hardly enough to worry about. How you get around this is use a camshaft with minimal overlap so said backpressure is used to drive the turbine and does not effect the intake charge. Also remember turbos while being driven by the exhaust are not solely a "fan" of sorts. The rapidly expanding thermal energy has a far greater effect than sheer air volume. Case in point, take any turbocharged engine, rev it up to say 2000 rpm below red line and hold it there. Now see how much boost it makes, chances are it will be very minimal. Now drive the car on the road, you can lug it in a high gear at low rpm and it will be spooling at or very near the maximum setting. Woah, makes more boost with less air being pumped through the engine...

    Yes turbos create a bit of heat on the turbine side, but with thermal coatings, and a water cooled housing that can be reduced a great deal. Also any serious application, be it SC or turbo will have an intercooler, so it's a rather moot point.

    I don't have all the figures here in front of me, but do a little research. Turbos are FAR more efficient than any mechanically driven compressor. If that weren't the case then why are turbos generally outlawed, or suffer a penalty when used against superchargers?
     
  14. ROGUE GTS

    ROGUE GTS Formula Junior

    May 24, 2004
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    As for the mercedes turbo thing Click Here to see your wrong

    Amazing how you can have an SL65 luxo barge, and with a chip change it will make 150hp and 300ft/lbs MORE than the SLR "supercar" Stock for stock it (the SL) makes 15 less hp but near 150ft/lbs more torque.

    Superchargers are just easier to tune, it's taken a while for the technology to advance to a level where turbos can be used effectively. That time is here, and as you will notice there are far more turbo cars on the market than SC.
     
  15. mk e

    mk e F1 World Champ

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    Fair enough, mine eats about 80 hp...with a 12 rib belt as tight as I can keep it and I need at least 210 degrees of wrap on a 3.5" pulley.

    I don't thing any of this is true really. The compressor for either a turbo or a SC require the same power to do the same work. It is easy to see on a SC, just look at the belt. With a turbo the work is not not directly by crank, it is done by the turbine, and maybe that's what you meant.

    Because the work is done by the turbine, it isn't obvious that it's there but it is. The turbocharger is essentailly a gas turbine engine with an external combustion chamber (the piston engine) and it is self-substaining depending on the rpm/boost level. any extra energy it requires is supplied by the pistons combressing the exhaust gas, any extra energy it could produce is dumped at the wastegate.



    Because the turbine is adding work required to run the compressor, in theory a turbocharged engine should able to make 5%-11% (your numbers) more hp than a supercharged engien at the same boost. In practice it is about 1/2 that because the back-pressure caused by the turbo prevents complete ylinder purging and therefore complete cylinder filling. There won't be any desimatng going on with either system, the SC car will pull away a bit off the line and the turbo car will catch back up at some later time.

    A positive displacement supercharger will make good boost at any rpm with absolutely no lag regardless of how much total boost the system is designed for. That is definately not the case with a turbo system.

    Turbo lag is very real, but honestly only relavent these days to higher boost systems. Definately not important in roadracing or most any racing, but it is however important on a street car where it's impossible to stay at ideal rpm and throttle.

    Honestly though what most people are really talking about when they say lag is boost threshold. By that I mean the rpm at which the engine makes at least 1/3 of it's total boost. With a supercharged engine, that point is idle so there is no rpm wher you push the gas and don't feel lots of torque. With a turbo car, depending on the total boost the system has, the boost threshold is 1/3 to 2/3 redline. what that means is that when you are just cruising along and stomp on the throttle, there is a very very noticable lag until the rush of power is felt. That "lag" isn't truely turbo lag (the time is takes to spool the trubo), it is the time it takes for the car to accelerate to the point that the engine has crossed the boost threshold, boost is available and the rush of the torque increase is felt. Down-shifting to the propper gear would solve it, but it is the street and you are often not in the right gear for best performance.
     
  16. mk e

    mk e F1 World Champ

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    That is true, a cooler charge will allow higher compression ratio on any engine, turbo SC or even naturally aspirated. But the intercooler does 2 other things. The first is it reduces the work require by the compressor to increase the density of the charge by any given amount, therefore increasing the efficiency and therefore power output. The second thing it does is allow a higher density charge to be send to the engine without causing detonation, so again the result is more power.

     
  17. bill365

    bill365 F1 Rookie

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    Mark,
    Please be patient with me, I am trying to understand what you are explaining.
    I have spent a portion of my day calculating orifice flow plates, for measuring the differential pressure created across it, to ascertain the fluid flow rate in a section of pipe.

    First you have to present a restriction to the exhaust flow to create the difference in pressure, then partially relieving the restriction achieving the increase in speed of the flow. High-pressure/low-speed vs low-pressure/high-speed (only true in compressable compounds). An orifice element obstructing the flow, creates a differential pressure and a corresponding percentage loss of efficiency.

    The only benefit from this expansion that you have discussed, is to help the turbo spin, so it's a non-issue in any other application? Additionally, the expansion ratio, because you are first restricting the flow of the exhaust gasses to then induce the expansion, requires the turbo to exert more force to get the fresh intake charge into the cylinder (in the valve overlap period), to overcome the back pressure created by the raised back pressure. You imply that the expansion ratio, is a greater benefit than the obstruction and high back pressure in the exhaust. I think some might argue that the open exhaust with the SC is better, not having to overcome the expansion ratio.

    I know that this is not really our realm of endeavor here but, why would top fuel cars not use turbos? Lag is not an issue with them, as they start the race with the revs up, they can run as large of turbos as they want. If there was an obviously greater efficiency found in the turbos, why SC's?

    In recent years, there have been more manufacturers involved in bringing SC's to market than ever before, in another thread it is even claimed that Ferrari is considering bringing a SC car to market, why?

    Regards,
    Bill
     
  18. ROGUE GTS

    ROGUE GTS Formula Junior

    May 24, 2004
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    Why not in topfuel? Have you looked at the NHRA or IHRA rule book? Your answer lies there, as in they are not class legal.

    Also I think you are confusing fuel with promod, as fuel cars leave the line at idle. There is no chance they would ever attempt to launch those cars at any increased rpm they will just blow the tires off. As it is the clutch doesn't lock up till nearly the 1000ft mark.

    Why a supercharger? Well could be for a number of reasons, if you want to preserve the ferrari exhaust note then a SC is the only way. That and from a packaging standpoint SC's are generally simpler.

    Look at the top of the line cars from the past 10 years, how many have had a turbo vs. SC.? Porsche has been running TT's for years, the F40s are TT, MB, and now almost all the aftermarket kits are turbo.

    Superchargers are old news, inefficient, and basically dated. If setup properly turbos can provide the best of most all worlds in the realm of a SC.
     
  19. 1Turbo

    1Turbo Formula Junior

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    100% Correct !

    I've had both SC & TT Cars. I'll take the Turbo please.
     
  20. 1Turbo

    1Turbo Formula Junior

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  21. mk e

    mk e F1 World Champ

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  22. mk e

    mk e F1 World Champ

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    I think that statement is rearly completely untrue. Turbochargers are more efficient when the engine is above the boost threshold, but below that they are 100% inefficient because they do nothing and a flow restriction to the engine on top of that. A single turbo can never ever span the engines full rpm band, 2 can come close, but 3 required to duplicate the performance of a positive displacement SC. That's not important on a roadrace track where engine rpm is alway in the top 1/3 of the band and why OEM makers who aspire to roadracing like them. for street driven cars, often a SC just makes more sense because of it's performace and simplicity over a sequentail 2 or 3 turbo system. I'm not saying there is anything wrong with trubos, just that everything has it's place.
     
  23. mk e

    mk e F1 World Champ

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    The turbine itself is the flow restriction. Anytime you reduce the pressure of a fluid, you can extract work. For example, in your home refrigerator there is an orifice (usually just a crimp in the tubing actually) at the exit of the condenser, it works, it’s cheap, it’s inefficient. In larger commercial units, that crimp is replaced with a turbine to pull work out, to help drive the compressor…it’s called a compressor-expander and in essentially a turbocharger. Now, just like your flow oriface work, the turbine is sized for a particular flow rate. At lower flow rates, the turbine is too small a flow restriction to generate adequate pressure for it to operate, that is why a turbo that works at high rpm can’t work at low rpm.

    The benefit of spinning the compressor is pretty big. It basically takes exactly the same power to spin the compressor of the turbocharger as it does to spin a SC making the same boost. So the turbine is doing all the work that the huge belt or the SC does. It either a turbocharged or SC engine, work goes into compressing the intake charge, but with a turbocharger, the turbine recovers the work.

    A turbocharger is a gas-turbine engine, it converts heat to work. The exhaust leaves a piston engine at something like 1500F, the turbocharger allows some of that waste heat to be converted to work. That is where the efficiency increase comes form.

    As a slight aside, to use all the heat, the compression/expansion ratio needs to be up in the 62 or 64:1 range. At that, the exhaust would leave the engine at ambient temperature the engine would hit something like 55% efficiency. If you do the compressor in stages with cooling in between, therefore reducing the work that goes into compression the efficiency could hit about 66%, which is the theoretical maximum for a fuel burning engine….not to be confuse with Carnot type efficiency which assumes temperature reservoirs.

    The added backpressure is really a net-zero, because the increased manifold pressure helps drive the piston down during the intake stroke. Where you see the effect is that all the exhaust can’t leave the cylinder, so the effective displacement of the engine is reduced slightly. The open exhaust of a SC is definitely better, but doen’t make up for the work gained by having the turbine and using the waste heat in the exhaust.



    4 reasons. First lag is an issue. It doesn’t matter what rpm you are at, if the engine isn’t under power, i.e. the throttle is open and air is flowing, the turbo cannot spool. A drag car sitting on the line doesn’t keep the turbo spooled. Second is a top fuel car is traction, not hp limited at basically all points on the track. Making more hp would do nothing to improve there times, it’s all about traction. Third is the torque curve and how it relates to traction. A smooth flat torque curve makes it possible to set up the clutch to keep the wheels at the limit of traction without spinning. If there is a step in the torque curve, like when a turbo spools or crosses the boost threshold, it would not be possible. Forth is a wide torque curve means less shifting. It takes time to shift and top fuel cars only need to do it once because they have a vey wide torque curve.


    Superchargers have become popular for several reasons I can think of. First is the hp war that has been going on for some 20 years now. The manufactures design a body to accept an engine and need to run it for about 8 year to make money, plus 2 –3 years for design, so they need to know what to make 10 or 11 years before it goes out of production. The manufactures often find that and engine is too big (70s) or too small (90s) during production. Too big is fine, they put in a smaller engine, but too small is a big problem when a bigger engine won’t fit. In that case forced induction or a high output engine are the choices.

    Then they look at cost. Normally a FI engine is cheaper to build than a very high output naturally aspirated engine…cost always wins.

    Once they decide they need forced induction, turbocharging was the only practical solution until the 80s. In the 80s eaton did 2 things, they build a blower with tight tolerances and no rotor seals so it spins free and doesn’t wear and they invented the by-pass valve which allows the blower to be unloaded. Those 2 things together mean that when boost is not required, the SC’s parasitic drag drops to about 1/3 mpg from 5-10 mpg and supercharging was practical.

    Now they have 2 choices, so they need to look at the market. Us drivers aren’t very accepting of narrow torque curves, so in the US supercharging sells the best (I own a turbo Jetta and it is my 1st and last Turbo car). And there you are, you now see SC’d cars.
     
  24. don_xvi

    don_xvi F1 Rookie

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    I haven't been paying much attention to this thread, but the reason turbos went out of favor in the 90s was emissions. The key to keeping up with emissions standards as they get tigher and tighter is catalyst lightoff. In a turbo application, you have an extra big hunk of cast iron in the exhaust that's got parts inside it that chew up the heat you need to get the catalyst up to temp fast. In the US auto industry, turbos were viewed as dead because of this. There are the handful of turbo packages out there because it CAN be done, but superchargers were seen as the only emissions-compatible forced induction for a long time. Note that it seems that the US industry was much more tied up in this view than the foreigners.
     
  25. ROGUE GTS

    ROGUE GTS Formula Junior

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    I completely disagree, This was true a few years ago but with the advances in turbo tech you can make the lag as non-existant as you want. If you look at ALL the supercharger systems, notice they incorporate a bypass valve to keep the engine out of boost when your not pushing it. With the advances in ball bearing turbos, thermal coatings, tuning, etc. you can build super efficient systems that will build boost literally too quick. Heck a car I was recently working on will start to spool backing out of the driveway, seems as though lag isn't going to be an issue there. Ohh and it's also running a pair of GT76's making over 1200rwhp, so not like there is some super tiny .5 a/r on there for it to spool so quick.

    Yeah if your running a 102mm thumper on a supra there is going to be some lag, DUH. But in that situation there isn't a supercharger on the planet that can support the HP it makes either.

    When it comes to a street setup the only excuse for lag is poor design. There are a few things you have to remember when saying boosted engines feel sluggish when not in boost. Obviously they will, 99% of the time they have a lowered compression ratio (both static and dynamic) this isn't best for running N/A as it makes less power.

    Also remember, when your trying to put power to the ground you don't want it to come in right off idle, it normally blows the tires off. Once you learn how to drive the car, be it stick or auto, you can put it in boost whenever you want.

    Go check around many different car forums and see whos making the most reliable, fastest, efficient FI setups, hint: They are ALL turbos. LSx guys have all but abandoned SCs in favor of turbos. Even the lightning and cobra owners are ripping the supercharger off and going turbo.


    Ohhh can you see the turbo lag

    Quite a few people are purposely oversizing turbos trying to raise the rpm level at which they spool, 800ft/lbs at 1800rpm is hard to put to the ground.

    So in the end, Turbos for the win, SC=old and busted.
     
  26. mk e

    mk e F1 World Champ

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    I’m sorry, I must be missing something, but this doesn’t make much sense to me. My comments had absolutely nothing to do with lag, any way…

    A by-pass valve on a supercharger system is there to prevent the supercharger from trying to create a huge vacuum between the SC and the throttle plate at partial throttle. It saves gas, nothing more, so I don’t understand what your point was here?

    There is no such thing at "building boost too quick". With a SC, the boost is instant and is completely linear with throttle position. That is not true with a turbo, that might be what you are talking about. With a turbo, at low rpm, full throttle might be fine, but as rpm builds and the boost threshold is crossed, the throttle often needs to be closed a significant amount and that can be quite difficult to control smoothly. On top of that there is some lag unless F=ma has changed recently. It’s not much on a good system, maybe .05 sec, but enough notice. So again I'm confused where you were going with this.

    There is a supercharger to make any hp or boost number you choose and they will do it with boost across the entire rpm band with instant linear responce. I don't know where you got the impression there was some limit.

    Well lag must be there unless physics has change, but I agree it isn't really a signifiacant amount on a well designed system. As I said before the problem is boost threshold.

    With boost threshold again physics jumps in and says that a turbine or centrifugal compressor will have an exponential flow/rpm relationship and a piston engine will be linear. That being true it is just impossible for a single turbo to make boost across the engines entire rpm band. The way a SC can. To get close at least 2 size turbos are require and 3 are require to match the boost curve of a SC. The lack of boost at low rpm is normally what people are actually talking about when they complain about turbos and they often incorrectly call it lag.

    The sluggishness is as you point out a lag of power or torque. With a turbo motor it is a real problem, but it doesn’t exist on a SC engine because opening the throttle always produces boost and the boost is instant and linear to throttle position. There is basically no difference in the performance of a supercharged engine vs a naturally aspirated engine of equal hp. None.


    Not a problem with a SC engine. The boost is linear with throttle position at every rpm. It doen't ever "come in" on it's own, it is simply a function of throttle position. yuo want less whell spin, launch at a lower rpm, there is no critical rpm and nothing with change as rpm builds. It is perfectly linear and predictable.


    There are a lot of centrifugal SC systems on the market and they are horrible, all my comment are about positive displacement SC systems. I wouldn’t have a centrifugal SC on any car I owned, that might be what you’re talking about here. I have never met a turbo owner that wasn’t literally stunned by the way an engine with a positive displacement SC performed on the street. I’ve personally had 3 friends sell their turbo cars or systems after riding in my car and replace them with SC cars or systems. Another 3 or 4 changed there plans and went SC instead of turbo after their ride. But again those were all street cars, the one still has a turbo roadrace car and that is probably how I would go too for the roadrace track.


    Again, just not a problem with a SC, on a street car, I'll stick with my SC.
     
  27. ROGUE GTS

    ROGUE GTS Formula Junior

    May 24, 2004
    827
    Kalifornia
    I'm not sure you understand turbos, they are generally not RPM dependant. You can rev the **** out of a turbo motor and build no boost, or have them spooling like crazy at 2k rpm. How they spool is directly proportional to load on the motor. Hence why you hear people talk about luging a turbo car to pull the turbo. And "lag" is just relative to efficiency, and somewhat a tuning aid depending on conditions. Hence boost controllers and the such.


    close but sooo wrong. Try to find a SC to fit a 3L supra that will make 1700hp. You would need a massive TS to support that kind of hp, and guess what, the internal friction in the SC would lend it completely useless. As well as the fact that they are EXTREMELY inefficient, produce way too much heat, etc. But you can drop a GT88 102mm turbo on a little 3L, and it will easily support 1600+ hp. And when it's not in boost does not have hardly any adverse effect on the motor.

    On the other end of the spectrum, big ci, viper V10, you can't get a TS big enough to run them much over 850rwhp. KB (autorotor) is the biggest at 2.8 or 3.2L, and they just won't do it. Great if you want something inexpensive, but they make less power than the paxton, and neither are with in 300hp of the basic turbo kits.


    When you say 2 or more turbos are required, are you talking about compounding them, or just running multiples? Cause having twins vs a single doesn't pan out. Twins are easier to package in all aspects, but efficiency wise it's really a moot point. And nobody compounds turbos on a street car, or hardly ever on race engines either. Thats more diesel stuff.

    And the centi compressor having an exponential flow/rpm ratio is correct, but the turbine rpm is not dependant on engine rpm. Remember turbos are not constrained to shaft speed that is proportional to engine rpm. You can have a turbo spinning 80k rpm when the engine is at 1800rpm, and it will maintain that rpm as engine speed increases. Wastegates are cool like that.

    thats somewhat true on a positive displacement SC, not entirely, but close enough.


    not exactly linear with throttle position, but with engine rpm. The way the bypass valves work is based on manifold vac, so you can have the engine turning 3k rpm but still making no boost. And with a turbo the boost threshold isn't a set in stone figure, there are ways around that. Changing timing and a/f ratio can drastically change how a turbo spools at low rpm ranges. And a well setup standalone will have the ability to adjust boost pressure in various gears, so it's really not a major issue if the turbo comes on like crazy. And like you said, if the turbo doesn't start to spool till 1800rpm, just launch at a higher rpm.



    Do you have any experience with centi supercharger systems? Honestly they do make more power on the top end compared to a TS, It's pretty simple to see that in just about every application around. Find an LSx and compare the magnusson setup to a procharger or paxton, not even a close comparison. I would quit studying the KB website so much, and take a look at what else is available.

    At any rate with a SC there is no getting around the excessive friction and HP loss just to turn the darn thing. I would rather not give up 50+hp just to spin the supercharger when a turbo will do it and eat maybe 10% of that. And thats if you can even get a SC big enough to fit under the hood. Sure you could attempt to bolt a 14-71 on a viper but that would be stupid, and thats how big of a setup you would need to make the power of a pair of GT76's.

    What exacly do you drive that has a TS on it? It's hard to compare different cars, but try to find a similar car setup with a turbo. Go check out www.turbomustangs.com there are quite a few 1000+ hp turbo 302's there that run low 5 sec 1/8th's, good luck convincing them that a TS will make more power anywhere in the rpm range.


    Take a look at these two dyno graphs. First being a basic bolt on Roe twinscrew, 2nd is an entry level TT kit. Both graphs come on very similarly, just that the turbo makes 300 more hp... hmmm, and yes they are both on pump gas and totally streetable.

    http://www.viperalley.com/attach/data/1/585541-610rwhpsmall.jpg

    http://www.heffnersperformance.com/images/SRT-10%20TT%20dyno.jpg

    Just to backup your claim, at 3000rpm where they started the pull the turbos had not fully spooled... but even still they made equal power, 400rpm later and the SC car is down by 150hp and 200ft/lbs, and it only continues to get worse from there on out. You can get a bit more power from the SC but it requires a meth injection setup to keep from blowing the motor up, and even then the SC it's self is pretty much spent by 750hp. Those turbos will support well into the 1200rwhp figure.

    So yeah enjoy those belt slippage problems, crazy intake temps, and giving up 50hp just to friction. All to make a little more power from idle to 2400rpm... yeah cause we all want huge boost at cruising rpm.

    At any rate, it's a pretty well known fact SC's are on the way out, heck even the ricer kids like turbos, lol.
     
  28. lusso64

    lusso64 Formula 3

    Apr 12, 2004
    1,535
    Simi Valley
    Full Name:
    David
    I have been following this thread with great interest, but now it seems to be getting a little personal. That's fine but I'd also like to know a little about the people involved - their background, experience, etc so I can better judge the validity of each argument.

    We all know what Mark drives and that he is a mechanical engineer. We have no idea about MXBLUE23 however, so please do us a favour and share a little about yourself - perhaps even fill out your profile.

    Dave
     

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